Reminds me of one of the stories my mother wrote years ago. In it, a customer asks a rude employee in a clothes store "Do you get paid a salary or by comission?"
"Salary. Why?"
"Then you can afford to scare away a customer."

Though I recall being forced to push something on people while working at McPaper. Had to sell whatever was the sale item at the register at the time. It wasn't a comission, I wasn't being paid more for selling it, but it was "Sell this, offer it to the customer when they're at the register, or you'll get fired".

The incentive is not getting poor performance reviews and/or potentially losing your job due to incompetence. A sales person's job, after all, is sales, whether or not they make commission.

I worked in a framing/crafts store for a couple years as a sales manager. I was consistently tops in the district in all key measurables, but my SM tried to write me up every month, because she made my numbers higher than some entire stores were required to hit. Why? So she could "reallocate" sales to her weaker-performing associates, who she protected. I had to print extra copies of all of my sales and take them home every night, otherwise I'd find a $1,000 framing job had been "adjusted" by her, and given to the weaker sales idiots. To this day, it's the only job I've ever had that I simply walked out of at the end of a shift and never went back.